Fluidized catalytic cracking processes operate by circulating catalyst particles continuously from a reactor in which a hydrocarbon feed is cracked to lower boiling products, during which cracking carbonaceous material is deposited on the catalyst, via a regenerator in which the carbonaceous material is combusted to restore the catalyst activity, and returning catalyst particles to the reactor. Temperature of combustion in the regenerator is locally dependent on the amount of carbonaceous material on the catalyst and the supply of combustion gas in a given region.
While operation with a single catalyst inlet opening at the regenerator side wall has for many years been satisfactory, the benefit to be obtained by improving radial distribution in conventional processes has become apparent. The benefits available with improved distribution of catalyst are particularly apparent for units comprising a regenerator vessel of increased diameter or in which regeneration is conducted at relatively high temperatures. Such is the case for example with the processing of residual feeds or where limited elevation space or specific operation mode require operation with a reduced regenerator fluid bed height, combined with a large bed diameter. A condition for optimum regeneration is that the time for radial mixing of catalyst be less than that for coke combustion. With relatively high regeneration temperatures and heavier feed processing the rate of coke combustion is increased, requiring decrease in radial mixing time. With increase in diameter of a fluid bed, catalyst distribution must be more effective to prevent corresponding increase in radial mixing time. Should this condition not be met, radial gradients of coke, combustion gas and temperature form within the bed leading to an increase in oxygen content of flue gas and afterburn and a decrease in coke burning capacity for a given air blower.
In U.S. Pat. No. 4,150,090 a device is disclosed comprising an axially located transport riser projecting from the lower part of a regenerator vessel and supporting a plurality of radially extending fluidized catalyst distributor troughs, located in downward sloping direction at the surface of a regenerator bed. Catalyst is transported and expelled along the length of the open-top troughs by means of fluidizing gas supplied via conduits running along the length of the troughs and having apertures along the length thereof.
In U.S. Pat. No. 5,156,817 devices are disclosed for supplying catalyst to one or a plurality of open-sided channels defined between a base and top member of, for instance, inverted v-shaped cross-section by which means catalyst is discharged along the length of the channel(s), the channel(s) being closed at their proximal end. A single channel forms an incomplete annulus in the regenerator bed. A plurality of channels are of different lengths and emanate in a fan formation from a supply conduit located towards the side of the bed, the longest channel extending to the axis of the regenerator bed.
These devices suffer from the disadvantage that with normal operation, catalyst discharge is uneven along the length of the channels or troughs, occurring to a lesser extent at the remote ends. Uniform discharge along the length of an open-sided channel would require excessive pressure drop which could be detrimental to the pressure balance of the unit. Provision of aeration conduits along distribution troughs incur high installation and maintenance costs. In all cases radial mixing is governed by interaction between the flow pattern in the channels or troughs and the fluidized bed, and is therefor sensitive to changes in flow rate in the distributor which may affect the quality of radial distribution.
We have now found that fluids introduction and subsequent mixing into a fluid mass can be attained in a simple and controllable manner which is moreover robust to changes in distributor fluid flow. This manner relies on discharging the fluid at specific points in the fluid mass while avoiding interaction between the bed and such points until the discharge point is reached. It has surprisingly been found that by this manner radial catalyst distribution gradients in the fluid mass are rapidly dissipated which would not have been expected. At the fluid discharge point the fluid kinetic energy may be locally eliminated after which the normal mixing action of the fluid mass promotes further radial mixing.